1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device provided with a circuit for controlling the contrast of a liquid-crystal display panel.
2. Description of Related Art
FIG. 1 is a circuit block diagram of the main portion of a conventional semiconductor integrated circuit device, showing the constitution of a single-chip microcomputer in which provided with a CPU, ROM, RAM, and other peripheral circuits are built. In the drawing, numeral 1 designates a semiconductor chip held in a package 2. In the semiconductor chip 1 are installed the following circuits: the CPU 3; a memory 4 consisting of such built-in memories as the ROM and RAM; a port 5 for inputting or outputting digital signals such as an input signal, which is inputted via, e.g., a keyboard, or an output signal to a calculating circuit; an LCD control circuit 9 for outputting an LCD driving signal so that it is given to an LCD panel (see FIG. 2); an LCD driving voltage generating circuit 7 for generating a driving voltage so as to drive the LCD control circuit 9; and a COM/SEG output circuit 11 for outputting the LCD driving signal to the outside. A clock generating circuit 6 generates clock signals CLK which are given to the CPU 3, memory 4, port 5, and LCD control circuit 9. The CPU 3, memory 4, port 5, and LCD control circuit 9 are connected to each other via an address bus 12 and data bus 13. The CPU 3 provides control signals 14, such as a read signal and write signal, to the memory 4, port 5, and LCD control circuit 9.
The LCD control circuit 9 is driven by the driving voltage generated by the LCD driving voltage generating circuit 7. Upon receiving the driving voltage, the LCD control circuit 9 outputs the LCD driving signal to the COM/SEG output circuit 11. The LCD driving voltage generating circuit 7 in FIG. 1 shows an example of a 1/5 biased operation, in which five resistances 8 having the same value of resistance R are connected in series between a reference potential V.sub.LCD for the LCD and the ground potential so as to generate six different driving voltages VL.sub.0 to VL.sub.5. The above reference potential V.sub.LCD is supplied from outside of the semiconductor chip 1 via a lead frame 17 attached to the package 2, wire 18 composed of a gold wire or the like, and pad 16 provided around the semiconductor chip 1. Output signals from the semiconductor chip 1 to the outside, including the LCD driving signal outputted from the COM/SEG output circuit 11, and input signals from the outside to the semiconductor chip 1 are all transmitted via terminals in the lead frames 17, wire 18, and pad 16, similarly to the reference potential V.sub.LCD.
FIG. 2 is a schematic diagram showing an example of the connection between the semiconductor integrated circuit device shown in FIG. 1 and the outside thereof. Among the large number of terminals in the lead frames 17, several are connected to the LCD panel 19 via a COM terminal 20 or SEG terminal 21, while others are connected to the power supply V.sub.CC directly or via a variable resistor 23. There is also another terminal in the lead frame 17 which is connected to the ground potential. To terminals in the lead frames 17 other than the ones mentioned above are inputted signals such as a reset signal and reference clock signal, but the description thereof will be omitted here.
Explanation will now be given to the controlling of the contrast of the LCD panel 19 by means of the semiconductor integrated circuit device thus constituted. The contrast of the LCD panel 19 changes in accordance with the voltage level of the LCD driving signal, i.e., with the reference potential V.sub.LCD for the LCD. When the voltage value is high, the contrast is also high. Conversely, when the voltage value is low, tile contrast is also low. To control the contrast of the LCD panel 19, therefore, it is necessary to change the reference potential V.sub.LCD. In the constitution shown in FIG. 2, for example, it is possible to change the reference potential V.sub.LCD by using the variable resistor 23.
With the conventional device thus constituted, it is necessary to provide a reference voltage control device, such as the variable resistor 23, outside the semiconductor integrated circuit device (single-chip microcomputer). This not only causes an increase in number of the parts required to fabricate a product to which the semiconductor integrated circuit device is attached, thereby increasing cost, but also is disadvantageous in terms of saving space.
On the other hand, to expand the range of applications for the semiconductor integrated circuit device, it is required to be versatile, for some products have no outside space sufficient for the provision of such a reference voltage control device as mentioned above, while other products have a sufficient space for the provision of the reference voltage control device. In the case where the number of the COM terminals 20 and SEG terminals of the LCD panel 19 is so large that it is difficult to control the LCD panel 19 by means of a single semiconductor integrated circuit device and it is necessary to use plural semiconductor integrated circuit devices, it is desirable to apply the same reference voltage to all the control circuits being used. Hence, there has been a demand for a versatile semiconductor integrated circuit device which is applicable to these various products.